Improved injection system for magnetic induction accelerators



Dec. 12, 1950 R. WIDERCJE 2,533,859

IMPROVED INJECTION SYSTEM FOR MAGNETIC INDUCTION ACCELERATORS Filed June 2, 1947 3 Sheets-Sheet 1 Dec. 12, 1950 R. WIDERGE 2,533,859

IMPROVED INJECTION SYSTEM FOR MAGNETIC INDUCTION ACCELERATORS Filed June 2, 1947 3 Sheets-Sheet 2 3 Sheets-Sheet 3 m, W M a R. WIDERGE IMPROVED INJECTION SYSTEM FOR MAGNETIC INDUCTION. ACCELERATORS Dec. 12, 1950 Filed June 2, 1947 Patented "Des 12, 1950 IMPROVED I NJECTION SYSTEM FOR MAG- NETIC INDUCTION ACCELERATOR/S Rolf Widcriie, Oslo, Norway, assignor to Aktiengesellschaft Brown, Boveri & (lie, Baden, Switzerland, a joint-stock company Application June 2, 1947, Serial No. 751,680 In Germany July 14, 1943 Section 1, Public Law 690, August 8, 1946 Patent expires July 14, 1963 22 Claims.

This invention relates to devices for accelerating charged particles such as electrons to high velocity and hence high potential by means of magnetic induction effects. These devices which are often referred to as betatrons or ray transformers include generally an evacuated toroidal tube into which streams of electrons are periodically introduced for acceleration, and a magnetic field system varying with time that has a direction transverseto the plane of the tube and electron motion.

The magnetic field affects the electron stream in two ways. Firstly, since the field has a direction transverse to the plane of motion of the electron stream, it tends to hold the electrons and cause them to follow a stabilized and generally circular orbit. In the second place, the timevarying magnetic flux enclosed by the orbit of any particular electron necessarily produces an electric field which tends to accelerate that particular electron along the orbit. During one cycle of operation, which will be explained in detail hereinafter, an incoming electron stream may be constantly accelerated for as many as several hundred thousand turns around the orbit and therefore reaches enormously high velocity and voltage, at which time the stream can be diverted from the orbit to produce useful results such as for example to bombard a target anode to create X-rays.

It has already been definitel established that the radius of th orbit in the tube along which the electron stream will be stabilized for acceleration will depend upon the distribution f the timevarying magnetic field with respect to the tube and such relationship may be expressed mathematically by the equation =21rr=Hr 1) where is the flux included within the electron orbit, r is the radius of the electron orbit, and

Hr is the magnetic field strength at the orbit.

It thus follows from this equation that electron stabilization will occur along an orbit at a radius r where the total flux within the orbit is twice that which would be produced by a homogeneous magnetic field having an average intensity equal to the field intensity at the orbit extending over the entire area enclosed by the orbit.

Using the above equation as a fundamental basis for engineering design data, it has proven practical to build a magnetic structure and to so locate the toroidal tube with respect to the magnetic field produced by the structure that the 2 equilibrium orbit along which electron acceleration is to take place will lie at any desired radial position in the tube, which is usually at the geometrical center. Suitable locations for the electron producing means and the target anode or exit point are then selected either radially inward of, or outward from, the acceleration orbit so that these elements will be entirely clear of any possible interference with the electron stream as it travels round and round in the tube along its orbit.

Once the electron stream produced by the electron gun has reached the equilibrium orbit of the particular radius for which the apparatus has been designed, no appreciable difliculties have been encountered in maintaining the stream on the orbit inasmuch as the magnetic structure is usually so designed that the strength of the magnetic field in the vicinity of the tube decreases radially outwardly at such a rate as to give rise to stabilizing forces which oppose any tendency on the part of the electrons to leave the main stream. Magnetic or electrostatic lens may also be used to augment stabilization of the electron streamat the acceleraion orbit.

However, considerable difficulty has been experienced in the past in introducing the electron streams into the tube in such manner that the streams will be. assured of reaching the acceleration orbit without impinging upon the walls of the tube or upon the electron producing means 'or the target anode assuming the device to be constructed with these elements located within the interior of the toroidal tube. If the electron streams are introduced into the tube from a point radially outward of the acceleration orbit, the time-varying magnetic field under which the stream is then accelerated does have some effect upon the stream to cause it to move along a spiral path of diminishing radius and gradually approach the acceleration orbit. However, under normal conditions, using frequencies of about 50 cycles per second for the magnetic field, the forces acting upon the electron stream for driving the manner that the electron gun and/or target anode or place of exit for the electron stream are. located outside of the bounds of the toroidal tube such as for example in arm portions which may object is to provide in an induction type electron accelerator for stabilizing or guiding an injected electron stream' along a spiral path into the acceleration orbit.

Another object is to provide for leading an electron stream into the acceleration orbit of an induction type electron accelerator along a spiral path, the guide path being established by a force acting radially on the entering electron stream and which diminishes rapidly with time following electron injection. Another object is to provide for gmding an electron stream of an electron accelerator into the acceleration orbit by means of a radially acting electrostatic field. Still another object is to provide a stabilized spiral entrance path for the entering electron stream of an induction type accelerator, 'the guide force being derived by disturbing the normal radial gradient of the time varied magnetic field. Another object is to provide for guiding an electron stream into a stabilized orbit for acceleration and for thereafter guiding the stream away from the orbit when the stream has reached the desired final velocity, the guiding means being produced by a radially acting force which is first applied when the stream is injected and subsequently reapplied when the stream reaches its final velocity. Yet another object is to provide for guiding an electron stream of an induction type electron accelerator by means of an auxiliary magnetic field which is superimposed upon the normal guiding force component of the main magnetic field, the auxiliary force serving first to guide the injected electron stream into the circular path along which its acceleration is to take place and also to thereafter guide the stream away from such path when the stream has attained the desired velocity.

A more specific object of the invention is to provide an auxiliary field producing device by which an electron stream when first introduced into an induction type electron accelerator is made to follow a contracting spiral path to carry the stream into the acceleration orbit, the path being thereafter expanded spirally by the same device when the electron stream has attained its final velocity to lead the stream outwardly from the orbit. Another specific object is to provide a stabilized spiral introduction path for the injected electron stream, the path being produced by a radially directed and rapidly decreasing electrostatic field that encloses a substantial portion of the circular path. Yet another specific object is to provide'for guiding an injected electron stream along a stabilized and rapidly diminishing spiral path into ultimate coincidence with the acceleration orbit by means of an auxiliary magnetic field which is superimposed upon the normal magnetic field employed for electron acceleration. Yet another object is to provide for guiding the electron stream along a path which is tangent to the spiral path by which the stream is thereafter guided into or away from the acceleration orbit.

These and other objects and advantages to be derived from the invention will become apparent from the following detailed description and from the accompanying drawings which illustrate several different constructional examples by which the desired improved results according to the invention may be attained.

In the drawings, Fig. 1 is a vertical central section through one constructional form of the invention; Fig. 2 is a horizontal section taken on lines 2-2 of Fig. 1 drawn to a somewhat enlarged scale, only the toroidal tube being shown; Fig. 3 is a substantially half vertical section showing a modified electrode arrangement for the Fig. 1 construction by which the auxiliary electrical field can be produced; Figs. 4 and 6 are also vertical central sections which illustrate other constructional forms of the invention; Fig. 5 is a diagram showing the control circuit used for the Fig. 4 construction; Fig. 7 is a somewhat diagrammatic view showing still another form of the invention; and Fig- 8 is a diagram showing the control circuit used for the construction shown in Figs. 1 and 2.

Referring now to the drawings, the induction accelerator shown in Fig. 1 comprises a magnetic field structure ill made up from vertically disposed steel laminations of appropriate contour which includes a pair of cylindrical pole pieces ll--l l' separated by air gap [2 and located concentrically along axis H, and a pair of annular poles li-II' facing each other and separated by air gap l4. Yoke members li coinplete the magnetic circuit for the time-varying magnetic flux set up in the annular and cylindrical poles. Poles H-ll' and l3l3' are surrounded by an annular winding preferably split into two coil sections i6-I6' wound in the same relative direction and connected in series for energization from an alternating current source ll of. suitable frequency which for example may be an alternator running at cycles/sec. An annular evacuated envelope l8, preferably glass, rests in the air gap l4 between the annular poles I3I3'.

By means of an electrical control circuit to be later described in more detail, electron streams are injected periodically into the tube l8 from an electron producing device 20 that is preferably located in an arm portion l8a of tube i8 arranged tangentially of the tube; Another tangentially arranged arm portion 18b of tube I8 is provided for example with a target anode 23 against which the electrons are caused to impinge to give off X-rays after the electrons have attained their final velocity. Once the injected electrons have reached the acceleration orbit K, the magnetic field varying with time inv poles llll and Iii-43' causes the electrons to be constantly accelerated round and round the tube along the orbit.

As previously explained, one of the principal difliculties encountered in the operation of these devices is properly guiding the electron stream from the electron producer 20 into the stabilized acceleration orbit K without loss, and in accordance with this invention, a most satisfactory guide path for the electron stream is established by applying an auxiliary and radially directed guiding force to the entering electron stream to augment the guiding or stabilizing force normally derived from the main magnetic field that produces acceleration of the electrons.

In the embodiment of the invention shown in Figs. 1 and 2, the auxiliary radially acting force for guiding the incoming electron stream is created by a radially directed electrostatic field set up between two sets or curved plates 24-24' and 25-25 located within the tube l8 and which together extend almost completely around the tube. The plates of each set are spaced parallel with and lie on opposite sides of the acceleration orbit K, and the plates extend in a direction parallel with the axis 11-11 of the magnetic induction apparatus. The electrostatic field produced between the plates of each set when charged thus extends in a direction radially of axis al-a.

In the illustrated construction of the magnetic induction apparatus in which the electron stream is introduced into tube l8 from a point radially outward from the acceleration orbit K, the auxiliary force to be applied to the incoming stream of electrons must of course be such as to guide the stream radially inward. The two sets of plates 24-24 and 25-25' are accordingly so charged that the potential of the outer plates 24 and 25 is positive with respect to the potential on the inner plates 24 and 25'. The plates are, charged prior to injection of the electron stream through a control circuit (Fig. 8) operating in timed relation with electron injection and the electrostatic field between the plates has its maximum strength at the time that the electron stream enters the tube. The electrostatic field is thereupon discharged by short-circuiting the plates through an impedance and the net effect of this diminishing electrostatic field on the the two sets of plates 24-24 and 25-25, it becomes possible to adjust the radial spacing of the sections a and b of the spiral curve so that even with a substantially high injection velocity of the electron stream, the latter may be guided safely into the acceleration orbit K without danager of electron loss by impingement upon the surfaces of the plates. The time constant may, for example, be of the order of 1 to 4580.

As an additional guiding force for the electron stream to enable it to reach the tube space between the arcuate condenser electrodes 24-24' and 25-25, another pair of electrodes 26-26 are provided immediately in front of the electron producer 20. These latter electrodes are charged to a constant positive voltage and the lectrostatic field set up between them guides the injected electrons along a path which is substantial- 1y tangential to the spiral path later traveled by J the electrons.

In order to deflect the electrons from the circular path after their acceleration has been completed, the second set of arcuate plates 25-25 is recharged at the proper instant to their original polarity to reestablish the electrostatic field. Hence, the electron stream will spiral outwardly from the acceleration orbit K at a rate determined by the rise in potential across the plates 25-25', and ultimately pass through another auxiliary guiding electric field established between positively charged plates 2'I-2'I' and impinge upon target anode 23.

The foregoing arrangement for deflecting the electron stream outwardly is however unsatisfactory to the extent that the electrons that are located between the deflecting plates at the moment of switching-in of the electrostatic field will describe a curved path different from the path of the electrons that are located outside the deflecting field at that moment and which are yet to pass between these plates. In order to keep the number of electrons traveling the first mentioned curved path as small as possible and to make them unimportant in th operation of the ray transformer, the field tor leading the stream away from orbit K, according to the invention, preferably takes up only a small part of the whole circumference of the circular path, about A or or even less. The portion or the electrons from the entire charge of the tube that follows a curved path difierent from that of the other'electrons is then correspondingly small and amounts only to A or or even less of the entire charge.

The set of deflecting plates 25-25 is accordingly made as short in the direction of travel (clockwise) of the electrons as is possible taking into consideration the necessary deflecting forces and the strength of the deflecting field to be produced. In order that in this connection, the

greatest'possible effect of the deflecting field shall I be obtained, the deflecting plates are arranged, according to a further feature of the invention, as far from the place of issuance of the electrons (or in front of the place where the defiected electrons are needed) as will give the greatest deflection. In the stabilizing field of force of the ray transformer, the electrons when deflected by a force transverse to the circular path will describe transverse oscillations at right angles to the circular path as is well known. The greatest deflection is given therefore approximately at a distance of a A wave length behind the center of the deflecting field. The best place for the deflecting plates 25-25 is thus about A wave length in advance of the place of issuance of the electrons.

The control circuit arrangement by which the two sets of plates 24-24' and 25-25' are initially charged prior to the injection of the electron stream into the tube, and thereafter discharged to guide thestream radially inwardly to the acceleration orbit K, and the control by which plates 15-25 are recharged upon completion of the acceleration phase of the stream to thereafter guide the latter outwardly from such orbit that is used in connection with another embodiment of the invention is shown in Fig. 8. In this diagram, 66 indicates a thyratron tube which switches the deflecting voltage on to the electrodes 25, 25 when the rotating electrons have attained the desired voltage and have to be brought out of the toroidal tube or made to impinge upon the target anode 23 as shown in Fig. 2. The instant at which this occurs can be adjusted by a phase shifter 12 which enables the phase of the thyratron grid voltage to be varied relative to the magnetic flux of the betatro n. Thyratrons 68 and 69, whose grids 58a and 59a re- N and 85, whilst the remaining elements shown in Fig. 8 have the same function as those described later on in connection with Fig. 5.

In accordance with a further feature of the invention,'the plates between which the electrostatic field is formed may be so constructed that they yield also a force component in the direction of the axis that has a stabilizing effect, i. e. it exerts forces on the electrons directed everywhere on the plane'of the circular path. In this way, the electron stream can be made smaller in the direction of the axis, this affordingmany advantagesfor the introduction and withdrawal of the. electrons, such as small admission and discharge openings, smaller losses by dispersed electrons and lesser influence of disturbing fields, etc.

With a field directed inwards, the strength of field consequently must diminish radially outwards,while with a field directed outwards, the strengthof field must increase in the radialdirection. A modified construction showing the latter case is illustrated in Fig. 3, the center line of the stabilized circular acceleration path of the electrons in tum l8 being designated K, and the arcuate deflecting plates shown at 30, 3|. A voltage forcing the electrons outward acts between the large inner plate 33 and the small outer plate 3|, i. e. the forces acting on the electrons have a component directed everywhere on the plane of the circular path. If the strength of the field is to diminish outwards, the larger plate 30 must then be placed at the outside and the smaller plate 3| at the inside of the annular tube l3.

In the event that it is desired to use electric or magnetic lenses for stabilizing the electron path, it may become impractical, because of spacial difllculties, to accommodate both the lenses and the electrostatic plates shown in Figs. l-3 which provide for introducing the electrons along a diminishing spiral path into the acceleration orbit. If the use of electrode plates is not practical then in accordance with another constructional embodiment of the invention as shown in Fig. 4, the radially acting guiding force which is applied to the incoming electron stream may be derived by disturbing the normal radial gradient of the main magnetic field in such manner as to temporarily alter the normal ratio between the magnetic flux within the tube l3, 1. e. the fiux through the inducing field poles H-H and the intensity Hr of the flux at the orbit K between the control field poles l3-i3'. Refer, ring now to Fig. 4, the general construction of the induction accelerator is seen to be similar to that shown in Figs. 1 and 2. However in lieu of the electrodes 24-24 and 25-25, auxiliary field windings are provided at the inner and outer sides of the control poles l3-I3'. One of these windings consisting of coil sections 32-32 is placed directly outside of the control poles l3-l3', and the other winding consisting of coil sections 33-33 is placed directly inside of the control poles. The two windings are connected in series for energization in a circuit which will be explained hereafter in detail, the current through one of the windings being in a direction opposite to that in the other winding.

Assuming the electron accelerating device is constructed in a manner similar to that shown in Figs. 1 and 2, in which the electron stream is introduced into the tube from a point which lies radially outward from the orbit K along which acceleration of the stream is to take place,

magnetic control field across poles i3-l'3f which is produced by the windings iG-IG' in order that the ratio at the control field with respect to the inducing field will be initially less than that at which the electron stream is stabilized at the radius of the orbit K. The eifect of the change in ratio between the two compo; cuts of the main magnetic field will be to enlarge the orbit of electron stabilization in the direction of the entrance path of the injected electron stream. As with the case of the electrostatic field produced by plates 24-24 and 25-25 of Fig. 1, the magnetic field produced by the auxiliary windings 32-32 and 33-33 must likewise diminish rapidly with time to zero so that the path of the injected electron stream is stabilized along a spiral orbit of decreasing radius as the normal gradient of the magnetic field is restored, the path ultimately merging with the acceleration orbit K when the field gradient has attained its normal characteristic.

It can be shown that the inducing path becomes a logarithmic decreasing spiral (i. e. the radius of the circular path changes according to the law R'=Ro+ARe-' when the auxiliary magnetic field changes according to the law:

where T is the time constant of the introduction path, e=2.783, and a and b are constants that r are dependent on special conditions (dimensions and frequency or the magnetic induction device).

Normally this time constant will be of the order of 1 to 10 p880 and since the winding by which the auxiliary magnetic field is produced may have large inductance (up to 1-2 millihenries) special consideration must be given to the design of the circuit for carrying out this switching operation fast enough. One suitable way for diminishing the auxiliary magnetic field with time would be to send a current through the auxiliary windings in the opposite direction and an arrangement for reducing the field in such manner will be presently described.

As with the construction shown in Fig. 1 and Fig. 8, the action of the auxiliary magnetic field must be timed with the injection of electrons into the tube I3 and energization of the windings l6-I 6' by which the magnetic inducing and control fields varying with time are produced. A suitable circuit is shown in Fig. 5. I

Referring now to Fig. 5, it will be seen that a blind load condenser 38 is connected in parallel with the main windings I 6-I6' across the terminals of the alternating current source I! and that a current transformer 39 which may preferably be of the highly saturated type has its pri mary winding connected in series with the main windings Iii-l6. The electron emissive device 20 by which streams of electrons are periodically introduced into the tube i8 for acceleration is supplied with high voltage from atransformer 43 also connected to the alternatin current source I! by way of a grid controlled vacuum valve ll. The grid a of valve (I is temporarily driven positive from an otherwise negative value sufficient to maintain the tube non-conductive by the voltage peak of the current transformer 33 whose secondary winding is connected between the cathode Alb and grid a of valve 4|, and the resultant current flow in the anode-cathode circuit of valve 4| causes a stream 01 electrons to be produced at the electron emissive device 23. After a brief period, a glow tube 42 connected in parallel with the secondary or transformer 33 becomes conductive and short circuits the same. Negative cut-oi! bias to grid 4la is then reestablished through grid battery 4Ic whereupon valve 41 is rendered non-conductive with the result that the production of electrons by the device 23 then ceases. Assuming the electron stream to be accelerated for a quarter oi a cycle of the magnetic flux, operation of the electron producing device 20 is preferably timed with energization oi themain magnetizing coils l6-l6' in such manner that the stream of electrons is shot into the tube II! at or near the instant at which the magnetic field produced by'windings l6-l6' passes through the zero point in the fiux cycle.

During the quarter cycle preceding introduction of the electron stream into the tube l3, the auxiliary windings 32-32 and 33-33' are energized through a resistor 31 and a rectifier 36 that is rendered conductive during this portion of the cycle, and the magnetic field produced by these auxiliary windings which threads across tube l3 through the control poles 13-13 opposes the main flux which is set up in these poles as a result of energization of the main windings Iii-l6. Rectifier 36 may as shown in the cirdisturbing field produced by the auxiliary windings 32-32 and 33-33 is already present in the 3 control poles l3-l3' and, since this disturbing magnetic field is opposed to the field established through these poles by windings l6-I6' the effect of the disturbing field is to decrease the normal ratio between the flux inside of orbit K and the field strength H1- at such orbit with the result that the incoming electron stream will be initially stabilized along it entrance path which lies radially outward of the acceleration orbit K. In order to gradually contract the entrance path of the electron stream inwardlyalong a spiral path which will finally merge with orbit K, the opposed magnetic field produced by auxiliary coils 32-32 and 33-33' is made to diminish with time to zero. In the present constructional embodiment of the invention, the opposing magnetic field is so reduced by sending a current through the coils in a direction opposite to that in which these coils are energized from rectifier 36, and such current is supplied to the coils by means of a circuit that includes a thyratron valve 34, resistor 44 and another section 600 or the secondary winding of transformer 60, Since the opposing field set up in the coils 32-32 and 33-33 is to diminish beginning with the instant that the stream of electrons is introduced into the tube, provisions are made for rendering the thyratron valve 34 conductive at such time and hence the grid 34a of thyratron 34 which controls its conductivity is preferably connected to the secondary of an air core transformer 43, the primary of which is connected in the anode-cathode circuit of valve 41 that controls the production of the electron stream. Thus as soon as valve M is rendered conductive to produce the electron stream, the accompanying voltage surge at the transformer 43 causes the grid 34a of thyratron 34 to swing to such potential that the thyratron in these coils by way of rectifier 36 and resistor 31 is neutralized with time by the reverse current flow through them the ratio of the field strength Hr at the orbit K to the flux within the orbit increases with the result that the entering path taken by the electron stream will be correspondingly decreased in a radial direction along a spiral path and finally coincides with the acceleration 1 orbit K when the opposing magnetic effect set up by the auxiliary coils 32-32 and 33-33' has been reduced to zero.

As the magnetic field produced by the main windings I6-l6' increases from zero with time, the electron stream introduced into the tube along the radially inwardly directed spiral path will be accelerated along orbit K. After the stream has been accelerated to the desired velocity, which in the constructional embodiment shown will reach a maximum when the flux produced by the main windings l6-l6' reaches its maximum, the auxiliary coils 32-32, 33-33' are again energized in the same direction as "they were energized from the rectifier 36 to thereby again set up an auxiliary and opposingmagnetic flux through the control poles i3-l3' that once again disturbs the normal ratio between the flux it within the orbit K and the field strength Hr at the orbit. Hence the stabilization orbit of the electron stream is now expanded radially outward from the acceleration orbit K to thereby guide the electron stream in a stabilized manner along a spiral path to the positively charged deflecting plates 21-21 provided at the-outlet arm 18b of tube l3.

The current for reenergizing the auxiliary coils 32-32, 33-33' at the termination of the acceleration period may be obtained by means of another control circuit which includes rectifier.

49, resistance 50, another thyratron tube 35 and condenser 46. The instant at which the control grid 35a of thyratron 35 renders the latter conductive is controlled through a transformer coupling 45, the condenser 41 and an adjustable resistor 43. Rectifier 49 is rendered conductive during the period in which the electron stream is being accelerated in the tube and charges condenser '46 through the resistor 6|. Thus when the acceleration of the electron stream is approaching completion and thyratron 35 is rendered conductive, condenser 46 is discharged through the auxiliary coils 32-32 and 33-33' which results in an expansion of the electron orbit.

Following the discharge of the electron stream from tube iii, the opposing magnetic field set up in coils 32-32 and 33-33' by the discharge of condenser 46 will disappear and the whole electron acceleration device will then remain inoperative for approximately that half of the flux cycle in which the flux changes between its maximum positive and negative values. Thereupon the rectifier 36 will again berendered conductive and the auxiliary coils 32-32 and 33-33 reenergized in advance of the introduction of the next following electron stream to be introduced into the tube l8 as the electron producing device 2|! is once again energized.

As may be seen from the Fig. 5 circuit, the time constant T1 for'the disappearance of the auxiliary'opposing magnetic field is determined by resistance 44 and the inductance L of the auxil iary coils 3232' and 33-33 1. e.

. -n. while the time constant T: for the rise of the auxiliary magnetic field used for ejection of the electrons from the acceleration orbit K will be determined by the resistance 50, i. e.

While the circuit shown in Fig. 5 is to be preferred; it is to be understood that it may be varied without departing from the invention.

In Fig. 6 there is shown an application of the invention to still another type of magnetic induction device for accelerating electrons. Re-

ferring now to Fig. 6, the magnetic structure of the device is seen to be similar to those shown in Figs. 1 and 4 except that the central core 5| is unbroken as distinguished from the pole pieces the introduction of the electron stream into the,

tube l8 by a control circuit similar to the one shown in Fig. 5 for effecting energization of the auxiliary coils 32-32' and 3333' of the Fig. 4 embodiment of the invention. The resulting voltage induced in the transformer secondary 53b causes an auxiliary magnetic flux to be set up in the magnetic structure of the induction accelerator prior tothe injection of the electron stream which disturbs the normal relationship between the inducing flux through the orbit K and the field strength Hr at the orbit. The disturbance has the same effect as in the other constructions and causes the normal control field to weaken with respect to the inducing field and establish an initial stabilization path for the incoming electron stream at a location radially outward from the acceleration orbit K. At the instant when the electron stream is shot into the tube, voltage of opposite polarity is applied to the primary winding 53a and the corresponding voltage induced in the secondary winding 531) then serves to neutralize the effect of the auxiliary flux previously set up by this winding. As with the other embodiments of the invention, the disturbing auxiliary flux diminishes rapidly with time from the instant of electron injection and hence causes the electron stream to spiral inwardly from the introduction path to the acceleration orbit K.

The invention may also be applied to the type of magnetic induction accelerator where the control poles of the device are pre-magnetized with direct current as more fully explained in my copending application Ser. No. 708,552, filed November 8, 1946. This type of device which has been illustrated in Fig. 7 somewhat diagrammatically is structurally similar to the device shown in Fig. 6 having an uninterrupted central core surrounded by a, winding ll correspondins to winding 52 of Fig. 6 energized in opposition to the main windings lt-IB', and a direct current winding I! on the control poles that produces a premagnetizing unidirectional component of flux in the control pulse. The alternating current that is induced in the direct current premagnetizing winding I! by the alternating component of the flux in the control poles must be compensated for in order that no alternating current shall flow in the direct current premagnetizing circuit. In the illustrated embodiment of the invention, the compensation is effected through the use of a transformer 56 having its secondary winding "a connected in series with the premagnetizing winding 55 and its primary 56b connected in parallel with the main field windings ll,l8' across the altemating current source I]. With this type of induction accelerator, the auxiliary magnetic field which is superimposed upon the control field through poles ll-i3' to establish the spiral guiding path for the incoming electron stream may be set up by means of an additional winding around the control poles but a preferred and more simple arrangement is to establish theauxiliary guiding field in the direct current winding 55 itself. One suitable arrangement as shown in Fig. '7 is to provide the compensation transformer 56 with an additional primary winding 560. i As with the other embodiments of the invention, the voltage applied at terminal 51' to this additional primary winding 560 would be .so phased in the operating cycle of the induction accelerator that the auxiliary and opposing flux set up by it in the control poles 13-43 would be established prior to injection of the stream of electrons, the flux thereafter being diminished with time. Also; as in the other embodiments of the invention. winding 560 may also be reenergized at the conclusion of the accelerating phase to lead the electron stream rect current pre-magnetizing. this additional transformer must likewise be provided with a suitable air gap.

In conclusion, it is to be understood that while in accordance with the patent statutes I have illustrated preferred constructional embodiments of the invention, the principles underlying the invention may be applied to types of electron accelerators in which the electron producing means and/or the target anode are located within the toroidal tube itself rather than in arm portions arranged tangentially thereto as illustrated. Furthermore, it will be evident that should the electron producing means be located radially inward from the acceleration orbit K, thus requiring an initial expansion of the entering orbit of the stream to reach the normal accelerating orbit, the auxiliary disturbing fielddecreasing with time which is superimposed upon the flux threading the toroidal tube across the control poleswould be such as to aid the flux through the control poles rather than op- .13 L pose it. If, in the latter construction. the target anode or place of exit for the accelerated electron stream is located radially outward from the acceleration orbit K, the auxiliary disturbing field to be produced at the instant of electron ejection trom the orbit K would be such as to oppose the field through the control poles, as described in the illustrated embodiments of the invention. 'Conversely, should the electron producing means be located radially outward from the orbit along which the stream electrons is to be stabilized for the acceleration phase and the target anode or exit pointbe located radially inward from the acceleration orbit, it then follows that the auxiliary field should first oppose the control field in order-to stabilize the incoming electron stream along a spiral path of decreasing radius to guide it into I the acceleration orbit, and to then augment the effect of the control field with respect to the inducing field when the stream has attained its ejection velocity in order to stabilize the electron stream along a spiral path further decreasing in radius so as to impinge upon the target anode.

For similar reasons, the polarity of the electro static field produced by the electrodes 24-24 and 25-25 in the Fig. l embodiment will of course depend upon the relative radial disposition of the target-anode and also theentering place of the electron stream with respect to the acceleration orbit.

' I claim:

1. The combination with an electron accelerator of the type wherein the electrons are caused to be accelerated along a stabilized orbit and wherein electrons are injected into the accelerator from a source spaced radially from said orbit, of means establishing a force acting radially on the electrons along substantially the entire periphery of said orbit prior to injection of the electrons, and means for diminishing said force rapidly with time at the instant of electron injection to thereby guide the injected electrons along a spiral path into said orbit.

'2 The combination with an electronaccelep ator of the type wherein theelectrons are caused to be accelerated along a stabilized circular orbit under the influence of a time varied magnetic field linking the orbital path of the electrons, and whereinthe electrons are injected periodically from a source spaced radially from sa d orbit in timed relation with the variation in said magnetic Y guide the latter alon a sp ral path into said stabilized orbitl 3. An electron accelerator as defined in claim 2, and further including means actuated in timed relation with the main time varied magnetic field for reestablishing said radially acting guiding force upon completion of the acceleration of the electrons to thereby gu de the electrons spirally away from the stabilized orbit.

4. A device for accelerating electrons as defined in claim 2wherein said radially actingforce for guiding the electrons is produced by an auxiliary magnetic field superimposed upon the than time varied magnetic field and which alters the normal gradient of the magnetic field to thereby shift the normal radius of the stabilized orbital 14 l a path 01' the electrons radially in the direction of the source 01 the electrons.

5. A device ioraccelerating electrons as defined in claim 4 and further including means thereafter operated in timed relation with said main magnetic field tor reestablishing said auxiliary magnetic field upon completion of acceleration of the electron stream, the auxiliary field increasing rapidly with time to guide the stream spirally away from the acceleration orbit.

6-. An electron accelerator as defined in claim 2 wherein the stabilized orbit lies in the air gap between juxtaposed annular poles, and the radially acting guiding force is constituted by an auxiliary magnetic field produced on at least one ofsaid poles.

7. An electron accelerator as defined in claim 2 wherein the stabilized orbit lies in the air gap between juxtaposed annular poles, and the radially acting guide force is constituted by an auxiliary time varied magnet'c field produced by a pairof windings, said windings being located outside and inside respectively 01' said annular poles.

8. An electron accelerator as defined in claim 2 wherein said radially acting guiding force is constituted by a radially directed electrostatic field set up between a pair of arcuate electrodes arranged parallel with and on opposite sides of the stabilized circular orbit.

9. A device for accelerating electrons as defined in claim 8 and further including control meansv operated in timed relation with said time varied magnetic field for subsequently reestablishing said electrostatic field to guidethe accelerated electron stream away from the acceleration orbit.

10. A device for accelerating electrons as defined in claim 8 wherein the electrode located radially outward from the acceleration orbit is of smaller dimension in an axial direction than the radially inwardly located'electrode.

11. A device for accelerating electrons as defined in claim 8 wherein said electrodes for establishing said electrostatic guiding field substantially enclose the acceleration orbit and are divided circumferentially into twoelectrode sets, one set' covering between A, and 1 s of the path, and further including means operated in t'med relation with the magnetic field for reestablishing an electrostatic field increasing with time between the latter set of electrodes upon completion of electron acceleraton for guiding the electrons spirally away from the acceleration orbit.

12. A magnetic induction device for accelerating charged particles such as electrons to high velocity comprising, a magnetic field structure includ ng a central core surrounded by a pair of juxtaposed annular poles, an evacuated annular tube located in the air gap between said annular poles, at least one magnetizing winding surround-- ing said control poles and central core, at least one auxiliary winding surrounding only said central core, means for cyclically energizing said windings to produce a time varied magnetic field in said central core and control poles, the magnetic flux produced in said central core by said auxiliary winding being opposed to the :ilux set up in such core by the other winding to establish a normal ratio between the flux in the central pole and "the control poles by which a stabilized circular path of predetermined radius for electron acceleration is provided, means spaced radially from the stabilized acceleration path and operated in t'med relation withenergization of said magnetizing windings for injecting an electron stream into said tube, and means operated byawinding ama s in timed relation with electron injection to alter,

the normal counter-magnetizing eiiect of said auxiliary winding, said lastmeans having a maxi-' acceleration path in the direction of the means for injecting the electron stream, which effect is tion device for accelerating charged particles" such as electrons along a stabilized circular orbit of predetermined radiusin an evacuated chamber under the influence of asymmetrical time varied magnetic field linking the orbit and arranged perpendicularly thereto, inwhich the radius r of the stabilized orbit is determined in accordance with the equation where is the magnetic-flux within the orbit and Hithe intensity of the field at the orbit, and in which the saidelectrons are periodically injected into said chamber in streams from an electron source spaced radially from the acceleration orbit in timed relation with the variation in said magnetic field, of means operated in timed relation with electron injection for guiding the stream along a spiral path into said acceleration orbit, said guiding means comprising a force,

acting radially upon the entering electron stream and which diminishes rapidly with time from the instant at which the electron stream is injected.

15. A magnetic induction device for accelerating charged particles such as electrons and the like to high velocity, comprising, an evacuated chamber providing an orbital path for electrons introduced therein, means adjacent said chamber for producing a main time varied magnetic field symmetrical with and perpendicular to the plane introduced therein, mea

of said path, said field linking said path and having a gradient in the plane of the path such as to normally establish a stabilizedcircular orbit of predetermined radius along which the electrons are accelerated under the influence of said field, means spaced radially from said stabilized orbit and operated in timed relation with said main field porducing means for injecting an electron stream into said chamber, and means operated in timed relation with electron injection for producing an auxiliary time varied field that is superimposed upon said main field to temporarily alter the normal gradient thereof, said auxiliary field being at its maximum value at the instant of electron injection to thereby shiftthe normal radius of the stabilized orbit in the direction of the initial path taken by the injected stream of electrons and thereafter diminished with time to establish a stabilized spiral path guiding the electron stream into said normal stabilized orbit upon restoration of said main field to its normal gradient.

16. A magnetic induction device as defined in claim 15 wherein the means producing said auxiliary time varied magnetic field is comprised of a winding energized in advance of electron injection by a current having a predetermineddirection through the winding and said auxiliary field is diminished by subsequent reenergization of said winding by a current of opposite direction. a.

, 1'1. A magnetic induction device as defined in claim 16 wherein the current for reenergizing the winding to diminish the auxiliary magnetic field is derivedfrom a source of current connected to the winding through a grid controlled discharge valve, and the potential on the grid of said valve is controlled in timed relation with operation of said electron injection means to render said valve conductive simultaneously with injection of the electrons.,

18. A magnetic induction device for accelerating charged particles such as electrons and the like to high velocity, comprising, an evacuated chamber providing an orbital path for electrons adjacent said chamber for producing a t e varied magnetic field symmetrical with and perpendicular to the plane of said path, said field linking said path and having a gradient in the plane of the "path such as to normally establish a stabilized circularpath of predetermined radius along which the electrons are accelerated under the influence of said field, means spaced radially from the stabilized acceleration path and operated in'timed relation with the field producing means for injecting a stream of electrons into said chamber, a pair of electrodes arranged parallel with the accelerationpath and disposed radially inward of and outward from said path, respectively, means operated in timed relation with electron injection for establishing a radially directed electrostatic field between said electrodes, said field being at a maximum at the instant of electron injection, and means for diminishing said field rapidly with time to thereby establish a stabilized spiral path for guiding the injected electron stream into the stabilized acceleration path. k

19. A device for accelerating electrons as defined in claim 2 wherein said radially acting guiding force is constituted bya generally radially directed electrostatic field set up between a pair of arcuate electrode plates arranged parallel with and on opposite sides of the stabilized circular orbit, one of said platesextending for a greater distance than the other in the direction of the axis of the orbit to thereby introduce an axial component in the gradient of said electrostatic field.

20. A device for accelerating electrons as defined in claim 2 and further including an electric lens disposed adjacent the electron source for initially guiding the electron stream emitted therefrom.

trol poles effecting acceleration of the electrons along said orbit, means for injecting electrons periodically from a source spaced radially from said orbit in timed relation with the variation in said magnetic field, an auxiliary winding on said control poles energized with direct current for premagnetizing said control poles, and means actuated in timed relation with said electron in- ,iecting means for impressing an auxiliary time varied current on said auxiliary winding to produce a time varied magnetic field force acting radially on the electrons along substantially the entire periphery of said orbit, said force being diminished with time simultaneously with in- Jection of the electrons thereby to guide the latter along a spiral path into said orbit.

22. A magnetic induction device as defined in claim 21 wherein the means for impressing the auxiliary time varied current on said auxiliary 1! winding is comprised of a transformer, said transformer including a secondary winding connected in circuit with said auxiliary winding, a primary winding connected in circuit with said magnetizing winding and the means energizing the same for compensating the potential induced ROLF WIDERdE.

REFERENCES CITED The following references are of record in the file of this patent:

UNITED STATES PATENTS Number Name Date 2,103,303 Steenbeck Dec. 28, 1937 2,193,602 Penney Mar. 12, 1940 2,394,070 Kerst Feb. 5, 1946 2,394,072 Westendrop Feb. 5, 1946 Kerst et a1. Aug. 1'7, 1948 

